This invention relates to bearings and more particularly to bearings for liquid metal pumps.
In liquid metal fast breeder nuclear reactors, a pump circulates a liquid metal in heat transfer relationship with a core disposed in the reactor vessel for transferring heat from the core to equipment for generating steam. The steam may then be used to produce electricity in a commonly understood fashion.
Generally, the liquid metal pumps employed in liquid metal fast breeder reactors, employ radial bearings that are immersed in and lubricated by the liquid metal that is being pumped. Since the temperature of the liquid metal can exceed 1000.degree. F., material stability problems as well as thermal transient problems can produce differential size changes between the liquid metal pump bearing unit and the mounting structure for the bearing unit. The differential size changes can interfere with proper functioning of the bearings and thus render the pump inoperable.
Thermal transient events which cause rapid changes in the temperature of the liquid metal on the order of 200.degree. F. to 300.degree. F. within a few minutes are part of the design duty cycle for these liquid metal pumps. Due to the much lower thermal inertia of the bearing unit relative to that of the pump internals structure that supports the bearing, these thermal transient events can cause large temperature differences to be generated between the bearing unit and the support structure. Detailed thermal stress analysis of these transient conditions indictes that conventional bearing mounting designs which may be employed in lower temperature applications exhibit unacceptable stress levels when employed for the liquid metal pump application.
In addition, experience with liquid sodium pumps has shown that the dimensional stability of the materials of construction for liquid metal pumps can be significantly affected by long-term exposure to temperatures on the order of 1000.degree. F. It has been determined that metallurgical changes in the bearing structure can occur due to operation of the pump at temperatures near 1000.degree. F. These metallurgical changes can cause a decrease in the mounting diameter of the bearing support structure that exerts excessive force on the bearing unit and, in turn, can cause sufficient decrease in the bearing clearance to cause seizure of the bearing and failure of the pump. An elaborate and expensive material heat treatment process can be employed to minimize dimensional stability problems. However, it would be desirable to be able to avoid such expensive procedures.
Therefore, what is needed is a liquid metal pump having a bearing support structure that has sufficient lateral stiffness to support the pump shaft under operating conditions and having the capability of accommodating differential thermal expansion.